A lens is a device which alters the direction of travel of an electromagnetic wave as the wave passes through the lens. This change in direction is a consequence of a change in dielectric constant and hence refractive index encountered by the wave as it passes from an adjacent medium into and through the lens. Lenses for use at optical frequencies include those used in corrective eyeglasses, cameras, binoculars and the like. Lenses also find application in connection with high frequency electromagnetic waves, such as microwave frequencies.
An important subclass of lens is the gradient lens. An electromagnetic wave passing through a gradient lens encounters a varying dielectric constant. A Luneberg lens is a spherical gradient lens having a defined dielectric constant gradient and finds particular application in providing antennas with very wide fields of view. In particular, current interest in them is driven by the telecommunication projects based on low earth orbit (LEO) satellites, which travel much closer to the earth than geostationary (GEO) satellites and are, therefore, more suited for high data transmission rates and Internet access. However, the movement of LEO satellites across the sky makes them difficult to track and a standard dish antenna would need to move to stay in contact with the satellite. A stationary Luneberg antenna can xe2x80x9cseexe2x80x9d the satellite across the entire horizon. An ideal Luneberg lens has a dielectric constant at its outer surface of approximately 1.0 and a dielectric constant of about 2.0 at its central core. For cases in which the focal point is located away from the surface of the spherical lens modifications well known to those skilled in the art are required.
The physical fabrication of the Luneberg lens has proven to be a significant challenge, and numerous methods of fabrication have been developed. According to Luneberg""s teaching, the refractive index should vary continuously as a function of the lens"" radial coordinates However, no practical techniques have been suggested for fabricating a lens having a continuously varying dielectric constant. Accordingly, it is current practice to fabricate a lens by assembling layers of lens components of different dielectric constants to effect a step-wise approximation of the theoretical refractive index gradient. Among the deficiencies associated with current methods of production include the presence of air gaps between layers which exacerbates gradient discontinuities and wave scatter within the lens. Layers fabricated by gaseous expansion of a polymeric matrix exhibit non-homogeneous regions with varying dielectric constant thereby degrading lens performance. Further, the expanding agent, normally a volatile hydrocarbon, presents flammability and toxic emission problems during manufacture of the lens. Foams fabricated from air encapsulated in glass microspheres fixed in a solid resin matrix (commonly referred to as syntactic foams) do not provide material with a dielectric constant less than about 1.4, deviating significantly from the desired dielectric constant of 1.0 at the lens outer surface, and therefore degrading lens performance. Further, such lenses are heavy and tend to be characterized by relatively high dielectric loss. Alternative methods, while producing satisfactory results, have proven to be extraordinarily expensive. A method for producing a Luneberg lens which overcomes problems or drawbacks associated with current methods of production would represent a significant advance in the art.
The present invention relates to a lens of gradient dielectric constant and methods for the production of same. The lens includes an inner core comprising a cured or fused substantially homogenous blend of glass spheres, curable or fusible binder and, as needed to increase dielectric constant, a low-loss dielectric material. The inner core preferably has a dielectric constant of about 2.0, and an interstitial void volume (representing trapped air space between elements of the cured or fused material) of from about 30% to about 50%.
The lens also includes an outer shell layer comprising a cured or fused substantially homogenous blend of hollow glass spheres and curable or fusible resin. The outer shell layer preferably has a dielectric constant of about 1.0, and an interstitial void volume of from about 30% to about 50%. In addition, the lens includes a minimum of one, and preferably two or more intermediate layers, the intermediate layers having a dielectric constant falling between 1 and 2. The intermediate layers are comprised of a cured substantially homogenous blend of glass spheres, resin and, as needed to increase dielectric constant, a low-loss dielectric material. As is the case with the inner core and outer shell layers, the intermediate layer(s) have an interstitial void volume of from about 30% to about 50%.